docs: def get_parent(self) -> Optional['CentralDogma']:
          """Get the parent, or none if it's already a reference node.
  
          :rtype: Optional[CentralDogma]
  
          Example usage:
  
          >>> ab42 = Protein(name='APP', namespace='HGNC', variants=[Fragment(start=672, stop=713)])
          >>> app = ab42.get_parent()
          >>> assert 'p(HGNC:APP)' == app.as_bel()
          """
          if VARIANTS not in self:
              return
  
          return self.__class__(namespace=self.namespace, name=self.name, identifier=self.identifier)